Origin of selected group of vertices is in odd position, why?
From your screenshot I see that you have selected 147 vertexes.



But it's looks like your circle has 32 vertexes. So I conclude that some of vertexes are off screen. But if it's not so, I think some vertexes are "doubles", that means you have several vertexes on same position. In this case you can:


Remove double vertexes (menu W -> Remove Doubles) 
Or use Bounding Box center instead of Median point as a center of transformation (,):